Instrumental Investigation of Oxygen Isotopes in Human Dental Enamel from the Bronze Age Battlefield Site at Tollense, Germany
Total Page：16
File Type：pdf, Size：1020Kb
Instrumental investigation of oxygen isotopes in human dental enamel from the Bronze Age battlefield site at Tollense, Germany T. Douglas Price Laboratory for Archaeological Chemistry University of Wisconsin-Madison [email protected] Michael J. Spicuzza Department of Geoscience University of Wisconsin-Madison [email protected] Ian J. Orland Department of Geoscience University of Wisconsin-Madison [email protected] John W. Valley Department of Geoscience University of Wisconsin-Madison [email protected] Revised Manuscript Mar. 1, 2019 Abstract Oxygen isotopes were analyzed in human teeth dating to approximately 1250 BC from a Bronze Age battlefield along the Tollense River in northwestern Germany. Tooth enamel was sectioned, prepared, and analyzed using Secondary Ion Mass Spectrometry (SIMS) and Confocal Laser Fluorescence Microscopy (CLFM). The results of the study indicate that diagenesis has locally altered the tooth enamel. Brightly luminescing domains seen by confocal laser fluorescent microscopy are chemically changed in oxygen isotope ratios and elemental [Cl] concentrations. Values of 18O are up to 2.7‰ lower in altered domains. Thus, diagenetic changes are observed in enamel that is 3250 years old and has been waterlogged for most of its depositional history. We recommend studies of enamel in human teeth routinely evaluate the possibility of diagenesis. Keywords archaeology, human proveniencing, tooth enamel, oxygen isotopes, diagenesis, confocal laser fluorescence microscopy, secondary ion mass spectrometer, SIMS 2 Introduction This study concerns questions about the preservation and alteration of stable isotope ratios and chemical composition in tooth enamel. We first introduce the issues of enamel formation, diagenesis, and oxygen isotopes in apatite. Next, we describe the archaeological site at Tollense, discuss the distinctive nature of the deposits, and the original oxygen isotope investigation of tooth enamel from the site. We then outline new procedures for the analysis of oxygen isotopes in human tooth enamel. A brief description of the instruments that were used in the investigation of the samples follows. Next we describe the samples used in this study and the preparation procedures that were employed. The results of the instrumental investigation are presented in narrative, photographic and graphic form to document changes in the samples over time that we believe are related to diagenesis. Our study concludes with a discussion of the significance of the results and some suggestions for future research. Tooth Enamel Formation Human tooth enamel formation and growth follows known or predictable patterns with appositional layers deposited in onion-like fashion (Fincham et al. 1999, Kang et al. 2004). Enamel development and mineralization is carried out by cells called ameloblasts that form a single cell layer that covers the developing enamel and is responsible for enamel composition. These ameloblast cells move together to lay down a protein-rich matrix that provides a blueprint for crystal growth (Lacruz et al. 2017). This process begins at the tip of the tooth cusp and continues to the cervix at the enamel-root junction. Thus, the earliest enamel to form is at the cusp and the latest at the root. This layer of ameloblasts obtains nutrients and building materials from the blood stream in the dentine and builds lines of hydroxyapatite (Ca10(PO4)6(OH)2) on the enamel surface. Enamel is - - 2- largely composed of hydroxyapatite (with minor substitution of F , Cl and CO3 ) and contains no collagen. Once formed it is devoid of any cells, so it cannot remodel (Lacruz et al. 2017). Formation times for various teeth in human dentition are well documented in the literature of oral biology (e.g., Nanci 2008, 3 Hillson 2005, Manjunatha and Soni 2014, Schour and Massler 1941). The finished enamel has a characteristic prismatic appearance composed of rods (or prisms), each formed by a single ameloblast, running parallel to the length of the tooth (Nanci 2008, Raue et al. 2012). The process of formation leaves regular, incremental microscopic bands in the tissue, perpendicular to the elongated axis of the prisms, that are probably due to metabolic variation in the secretion of ameloblasts (Boyde 1976, Boyde et al 1988, Dean 2000, Risnes 1986). These variations produce short- and long-period incremental (growth) lines in the enamel (Mahoney 2008). Long-period growth lines are known as the striae of Retzius and form every 6-12 days during human enamel formation (Schwartz et al. 2001, Reid and Ferrell 2006). Smith (2006, 2008) has provided new evidence for the periodicity of these incremental structures. Antoine et al. (2009) presented convincing evidence that the short-term growth bands (cross- striations) reflect a circadian pattern of daily formation. Counts of these bands provide a chronology for reconstructing development in archaeological human dentition. Many studies rely on the periodicity of this cross-banding. Le Cabec et al. (2015), for example, utilized this information to determine crown formation time and age at death in juvenile fossil Australopithicine hominins from South Africa dating to more than 1 mya. One of the challenging aspects of enamel formation in humans involves sampling strategies for obtaining information on life history, especially diet and mobility. Although age of formation and development of the enamel can be determined, there are a number of problems remaining regarding the location of enamel of specific age that can be used to estimate a particular point in the early life of an individual. For example, determination of seasonal variation in diet or migration would be very useful information, but to date attempts to isolate enamel from such specific episodes of time have not yet been successful in humans. One problem lies in the technology necessary to locate and sample such minute areas of enamel; another major problem involves the appositional growth process whereby thin layers of enamel are deposited one on top of another over 4 time with intergrowth between the layers. Nevertheless, seasonality has been resolved in 18O of tooth enamel from large mammals (Kohn et al., 1998) and time-averaging was minimized by in situ SIMS analysis of aprismatic enamel < 20 m from the enamel-dentine junction in a laboratory-reared rat (Blumenthal et al., 2014). Tooth Enamel Diagenesis There are also potential problems due to diagenesis (e.g., Budd et al. 2000, Nelson et al. 1986, Pollard 2011, Schoeninger et al. 2003, Sharp et al. 2000, Shin and Hedges 2012), although little agreement on their scope. Renewed interest in the subject has followed the increasing number of isotopic studies of human remains (e.g., Balter and Zazzo 2014, Kendell et al. 2018). Most discussions have focused on carbon and strontium isotopes with relatively little attention to oxygen. There has been a general assumption that the hardness and impermeable nature of enamel retards or prevents contamination. Studies have produced various pro and con arguments. Enamel is clearly denser and more crystalline than bone or dentine and more resistant to post-mortem alteration (e.g., Hoppe et al. 2003, Lee-Thorp and van der Merwe 1991, Lee-Thorp and Sponheimer 2003, Sponheimer and Lee-Thorp 1999), but enamel does not always escape diagenetic change. Most examples of diagenetic alteration occur in enamel from ancient sites, greater than 1 m.y. in age, in contexts where skeletal remains were in the process of fossilization. Whether more recent human teeth undergo such significant change is unknown; diagenesis is rarely reported in such cases. A variety of chemical, instrumental, and imaging techniques have been applied to the question of diagenesis in human remains without firm answers. There are two general groups of such studies, one focused on modern teeth with an orientation toward dental issues and a second concerned with past dentition and aimed at archaeological questions. Most of these studies employ bone or enamel powder in the analysis rather than whole tissue. Rink et al. (1995) used Electron Spin Resonance to examine carbonate in tooth enamel and found limited evidence of 5 diagenesis (2 of 11 samples). Zazzo (2014) using radiocarbon to compare materials concluded that the 13C record measured in bone apatite is probably as reliable as that in enamel for at least the past 40,000 years. More recently, sampling has been done in three dimensions to try and resolve questions regarding diagenesis in tooth and bone. These studies focus on: micro-sampling or direct analyses using micro-ablation to study the distribution of elemental or isotopic values (e.g., Aubert et al. 2012, Brady et al. 2008, Cerling and Sharp 1996, Duval et al. 2011, Olivares et al. 2008, Reiche et al. 1999, 2002, Thomas et al. 2011). Al-Jawad et al. (2007) used micro CT scanning to look at enamel formation and lattice parameters. Similar techniques have been applied to archaeological teeth (Montgomery et al. 2012). Simmons et al. (2011, 2013) have used synchrotron X-ray diffraction to map the process of biomineralization and X-ray microtomography to study mineral content distribution in the formation of human enamel. Aubert et al. (2012), Blumenthal et al. (2014), and Beasley et al. (2017) used SIMS (Secondary-Ion Mass Spectrometer) to make high spatial-resolution in situ micro-analyses of oxygen isotopes in teeth and other materials. Lebon et al. (2014) utilized Attenuated Total Reflection - Fourier-Transform Infrared Spectroscopy (ATR-FTIR) mapping of bone to document better preservation in the center of cortical bone and taphonomical uptake of carbonate in the most external part of the bone. Ségalen et al. (2008) used cathodoluminescence to map trace element distribution in fossil remains. These studies demonstrated that different mineralized tissues (enamel vs. bone, dentine and cement) and their structure influence the uptake of trace elements during diagenesis (Gaschen et al. 2008, Hinz and Kohn 2010). In general, the range of variation observed in tooth enamel was much smaller than in other materials and argued for less diagenesis in the enamel.